1. Field of the Invention
The present invention relates to a light frequency control apparatus which is ideal for use, for example, in a frequency multiplex transmission system.
2. Relevant Art
In recent years, light frequency control devices for generating CW (continuous) light of a predetermined light frequency have been widely developed. A structural example of this type of device is shown in FIG. 6. As shown in this figure, a standard frequency light source 10 for outputting CW light possessing a light frequency fixed at a predetermined value; light switch 11 for converting CW light supplied from standard frequency light source 10 into a light pulse signal 11a; light frequency shifter 12 for shifting and outputting a light pulse signal 11a supplied from light switch 11; and timing control circuit 13 for respectively supplying a timing signal 13a to light switch 11, and a timing signal 13b to light frequency shifter 12, are respectively provided.
In light frequency shifter 12, a light coupler 12a for introducing light pulse signal 11a inputted via light switch 11 into EDFA 12b, and for inputting a feedback light signal supplied from frequency shifter 12f (to be explained hereafter) into EDFA 12b; EDFA 12b (erbium addition optical fiber amplifier) for amplifying a light pulse signal supplied from light coupler 12a; BPF 12c (band passing filter) for cutting out ASE (spontaneous light emission) of the light pulse signal supplied from EDFA 12b; light coupler 12d for outputting a light pulse signal outputted from EDFA 12b as light signal 103a (light pulse sequence), and for supplying a portion of this light pulse signal to delay fiber 12e; delay fiber 12e for delaying a light pulse signal for a predetermined delay period; and frequency shifter 12f for shifting the frequency of a light pulse signal, and supplying this resultant signal to light coupler 12a as a feedback light signal, are respectively provided.
According to this structure, light frequency shifter 12 outputs, at regular time intervals, a light signal 103a (light pulse sequence) in which the light frequency component is shifted to form a staircase state. Further details regarding this light frequency shifter 12 are disclosed in Shimizu, Kaoru, et al. ("Technique for translating light-wave frequency by using an optical ring circuit containing a frequency shifter"; OPTICS LETTERS, Vol. 17, No. 18: Sep. 15, 1992).
However, in this conventional light frequency control apparatus, in contrast to the high coherence displayed therein, light frequency shifter 12 exhibits a strong polarization dependency due to the polarization dependency possessed by delay fiber 12e, frequency shifter 12f, EDFA 12b and BPF 12c. Consequently, in light coupler 12a, when the inputted light pulse signal 11a and the feedback light signal circulating in the light frequency shifter are coupled, problems arise such as a reduction in the number of cycles due to a deterioration of the S/N ratio of the light signal.